The present invention relates to a system for dissipating heat from a high power density device (HPDD). More specifically, the present invention relates to a heat spreader that helps in effective dissipation of heat from the HPDD.
Electronic devices such as central processing units, graphic-processing units, laser diodes etc. generate a lot of heat during operation. In case the generated heat is not dissipated properly from high power density devices, this may lead to temperature buildup in these devices. The buildup of temperature can adversely affect the performance of these devices. For example, excessive temperature buildup may lead to malfunctioning or breakdown of the devices. So, it is important to remove the generated heat in order to maintain normal operating temperatures of these devices.
The heat generated by HPDD is removed by transferring the heat to the ambient atmosphere. As heat generated by HPDD increases, more heat has to be transferred to maintain the operating temperature of the HPDD. The transfer of heat from a HPDD to the atmosphere faces thermal resistance. In order to transfer more heat, this thermal resistance has to be reduced. One way to reduce thermal resistance is to increase the effective surface area of the hot device, for this purpose a finned heat sink structure is sometimes used. This finned heat sink structure increases the effective surface area for transfer of heat from the HPDD, thereby reducing the thermal resistance between a HPDD and atmosphere.
FIG. 1 shows a typical finned heat sink structure 101 mounted on HPDD 103. The HPDD is mounted on mounting board 105. Heat generated by HPDD 103 is transferred to finned heat sink structure 101. This finned heat sink structure in turn dissipates the heat to the atmosphere. The surface area of the finned heat sink structure is much larger than that of the HPDD. This increases the effective surface area for transfer of heat from the HPDD leading to reduced thermal resistance between the HPDD and the atmosphere. The reduced thermal resistance results in an increase in the rate of dissipation of heat to the atmosphere. Heat is transferred from the finned heat sink structure 101 to the atmosphere by natural convection, or by forced convection with the use of a fan.
However, the material used to construct finned heat sink structures has inherent resistance to the flow of heat. This leads to uneven distribution of temperature at base of the finned heat sink structure. This reduces the effectiveness of the finned heat sink structure in dissipating the heat.
FIG. 2A shows a finned heat sink structure 201 with uneven temperature distribution at the base of the structure. Arrows of differing lengths show the flow of heat in the finned heat sink structure. The shorter lengths of arrows at the outer fins depict less flow of heat as compared to the flow of heat in the inner fins (depicted by large arrows). The outer fins of finned heat sink structure 201 are not as effective in dissipating heat as the inner fins. This is due to an uneven temperature distribution at the base. FIG. 2B shows a finned heat sink structure 205 with uniform temperature at the base of the structure. The same lengths of arrows at the outer and inner fins depict uniform flow of heat in the fins. Thus, unlike finned heat sink structure 201 of FIG. 2A, the outer fins of finned heat sink structure 205 are as effective in dissipating heat, as its inner fins.
Hence, in order to increase the effectiveness of the finned heat sink structure heat needs to be uniformly distributed at the base of the structure. An important way for uniformly distributing heat at the base of the finned heat sink structure is by using a device called a heat spreader. The heat spreader is placed between HPDD and finned heat sink structure and spreads heat uniformly at the base of the structure.
FIG. 3 depicts the use of heat spreaders for uniform distribution of heat. FIG. 3A shows a HPDD 301 on a mounting board 303. A Finned heat sink structure 305 is placed in contact with HPDD 301. Heat is removed by conduction from HPDD 301 to finned heat sink structure 305. Heat is thereafter dissipated by finned heat sink structure 305 to the ambient atmosphere.
FIG. 3B shows a heat spreader 307 placed between HPDD 301 and finned heat sink structure 305. Heat spreader 307 increases transfer of heat between HPDD 301 and finned heat sink structure 305 by evenly distributing heat at the base of finned sink structure 305.
Heat spreaders are usually made of materials with low thermal resistance. Examples of such materials include copper or aluminum. Lightweight materials having high thermal conductivity such as graphite sheets and CVD (chemical vapor deposition) diamond thin film are also used for making heat spreaders. Typically, these high thermal conducting materials are costly and do not increase performance of the heat spreader substantially more than aluminum or copper heat spreaders.
Head spreaders may also be based on vapor chambers. A vapor chamber based heat spreader involves the vaporization and condensation of the liquid filling it. This heat spreader has a vacuum vessel with a saturated wick structure lining the inside walls. As heat is applied to the base of the heat spreader, the working fluid at the base immediately vaporizes, and the vapor rushes to fill the vacuum. Wherever the vapor comes into contact with cooler wall surface it condenses, releasing its latent heat of vaporization. The condensed liquid returns to the base via capillary action in the wick structure.
FIG. 4 shows a heat spreader 401 based on a vapor chamber. Heat spreader 401 is placed between HPDD 403 and finned heat sink structure 405. Heat spreader 401 has two surfaces, a surface 407 in contact with the finned heat sink structure 405 and a surface 409 in contact with HPDD 403. There is a lining 411 inside the heat spreader. Liquid on surface 409 absorbs heat from HPDD 403, evaporates and fills the vacuum in heat spreader 401. When it comes in contact with surface 407, it transfers heat to finned heat sink structure 405, condenses and moves back to surface 409 due to gravity or the capillary effect. The capillary action enables the performance of vapor chamber based heat spreader to be less dependent on the device's orientation with respect to gravity Also, thermal resistance associated with the vapor spreading is negligible.
However, maximum heat transfer in a vapor chamber based heat spreader is limited by vapor/liquid nucleation properties. Heat transfer is also limited by the presence of interface resistances such as that between metal surface and liquid layer and between metal surface and vapor.
From the above discussion, it is evident that presently available heat spreaders suffer from various limitations that limit the effectiveness of these heat spreaders. These limitations lead to higher device operating temperatures and decreased performance of HPDD. Thus, there is a need for heat spreaders that can effectively remove the heat from high power density devices.